Methods for Preparation of Concentrated Graphene Compositions and Related Composite Materials

ABSTRACT

A rapid, scalable methodology for graphene dispersion with a polymer-organic solvent solution and subsequent solvent exchange, as can be utilized without centrifugation, to enhance graphene concentration.

This application is a continuation of and claims priority to and thebenefit of application Ser. No. 14/797,999 filed Jul. 13, 2015 andissued as U.S. Pat. No. 10,030,161 on Jul. 24, 2018, which was acontinuation of and claimed priority to and the benefit of applicationSer. No. 13/453,608 filed Apr. 23, 2012 and issued as U.S. Pat. No.9,079,764 on Jul. 14, 2015, which claimed priority to and the benefit ofapplication Ser. No. 61/478,361 filed Apr. 22, 2011—each of which isincorporated herein by reference in its entirely.

This invention was made with government support under DE-FG02-03ER15457awarded by the United States Department of Energy. The government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

Graphene, a two-dimensional sp² hybridized lattice of carbon atoms, hasgenerated intense interest due to its unique electronic, mechanical,chemical, and catalytic properties. Recent synthetic efforts havefocused on the development of high-yield and scalable methods ofgenerating graphene. These techniques include the direct exfoliation ofeither chemically modified or pristine graphene directly into varioussolvents. For example, graphene oxide (GO) can be exfoliated fromgraphite via acidic treatments. The resulting GO flakes containhydroxyl, epoxyl, carbonyl, and carboxyl groups along the basal planeand edges that render GO strongly hydrophilic. The ease of dispersing GOin solution has facilitated the preparation of GO thin films andGO-polymer nanocomposites with interesting and potentially usefulmechanical properties. However, due to the defects and consequentdisruption of the graphene band structure introduced during oxidation,GO is a poor electrical conductor. Although the level of oxygenation canbe partially reversed through additional chemical reduction steps,significant quantities of structural and chemical defects remain.Moreover, the electrical conductivity of reduced GO flakes is less thanoptimal and is certainly deficient by comparison to pristine graphene.

In an effort to circumvent such GO limitations, recent efforts havefocused on direct solution-phase exfoliation of pristine graphene.Graphene sheets can be extracted using superacids, by sonication insurfactant solutions and through use of organic solvents. For example,superacids have demonstrated an unprecedented graphene solubility of 2mg/mL through the protonation and debundling of graphitic sheets.However, the resulting solutions are not ideally suited for additionalprocessing due to their acidity-dependent solubility and highreactivity. Direct exfoliation of graphene in surfactant solutions andselect organic solvents has also been demonstrated with concentrationsup to 0.3 mg/mL and 1.2 mg/mL, respectively, but such concentrations areachieved only following prolonged sonication times—approaching 3 weeksin duration—or extended ultracentrifugation.

Processing complexities represent a bottleneck for fundamental studiesand end-use applications that require well-dispersed, highlyconcentrated, pristine graphene solutions. Accordingly, there remains anon-going search in the art for an improved approach to graphene solutionconcentrations sufficient to better realize the benefits and advantagesavailable from graphene and related material compositions.

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide methods relating to the preparation of concentrated graphenemedia, together with corresponding compositions and composites availabletherefrom, thereby overcoming various deficiencies and shortcomings ofthe prior art, including those outlined above. It will be understood bythose skilled in the art that one or more aspects of this invention canmeet certain objectives, while one or more aspects can meet certainother objectives. Each objective may not apply equally, in all itsrespects, to every aspect of this invention. As such, the followingobjects can be viewed in the alternative, with respect to any one aspectof this invention.

It can be an object of the present invention to provide an economical,efficient approach to the preparation of graphene solutions andcorresponding graphene ink compositions at concentrations sufficient forend-use applications.

It can also be an object of the present invention to provide a rapid,scalable methodology for preparation of highly-concentrated graphenemedia without impractical, time-inefficient sonication andcentrifugation operations.

It can also be an object of the present invention, alone or inconjunction with one of the preceding objectives, to provide a rapid,scalable methodology which can utilize low-cost organic solventsotherwise deemed ineffective for purposes of graphene exfoliation and/ordispersion.

Other objects, features, benefits and advantages of the presentinvention will be apparent from the summary and the followingdescriptions of certain embodiments, and will be readily apparent tothose skilled in the art having knowledge of various graphenepreparation methods. Such objects, features, benefits and advantageswill be apparent from the above as taken into conjunction with theaccompanying examples, data, figures and all reasonable inferences to bedrawn therefrom.

In part, the present invention can provide a method of using acellulosic polymer for preparing concentrated graphene media and relatedcompositions. Such a method can comprise exfoliating a graphene sourcematerial with a medium comprising an organic solvent at least partiallymiscible with water and a cellulosic polymer dispersing or stabilizingagent at least partially soluble in such an organic solvent; contactingat least a portion of such an exfoliated graphene medium with ahydrophobic fluid component; and hydrating such a graphene medium toconcentrate exfoliated graphene in such a hydrophobic fluid component.Without limitation, such concentration can be achieved withoutapplication of centrifugal force.

In certain non-limiting embodiments, such an organic solvent can beselected from suitable alcohols, esters, amides, ethers, and ketones andcombinations thereof, such a solvent as can partially solubilize such acellulosic dispersing agent. In certain such embodiments, such a solventcan comprise ethanol or dimethylformamide. Regardless of solventidentity, such a dispersing agent can comprise a cellulose polymer about46-about 48% ethoxylated.

Without limitation as to identity of an organic solvent and/or adispersing agent, a hydrophobic fluid component of this invention can beselected from fluid hydrophobic components at least partially misciblewith such an organic solvent but immiscible with water. Such hydrophobiccomponents include, without limitation, chloroform, ˜C₆-˜C₈ alkanes,terpenes, terpene alcohols and combinations thereof. In certainembodiments, such a hydrophobic fluid component can comprise aterpineol. Regardless, such a method can utilize a graphite as agraphene source material.

Without limitation as to organic solvent, dispersing agent and/orhydrophobic fluid component, a method of this invention can compriseiterative separation of a graphene-hydrophobic fluid component from suchan organic solvent medium, and subsequent contact with another portionof such an exfoliated graphene medium. A resulting concentrated grapheneink can be applied to a substrate component, then can be annealed toremove residual dispersing agent.

In part, the present invention can also be directed to a method ofconcentrating a graphene medium. Such a method can comprise exfoliatinggraphene from a graphene source material with a medium comprising anorganic solvent selected from ethanol and dimethylformamide, and anethyl cellulose; contacting at least a portion of such an exfoliatedgraphene medium with a terpineol; adding water to the graphene medium toconcentrate exfoliated graphene within such a terpineol component;separating such a graphene-terpineol component from such a hydratedmedium; and, optionally, iterative contact of such a separatedgraphene-terpineol fluid component with additional portions of anexfoliated graphene medium, to concentrate graphene therein. Suchconcentration can be achieved absent centrifugation. A graphene inkresulting from such iterative concentrations can be applied to asuitable substrate, then annealed to remove dispersing agent.

Accordingly, the present invention can also be directed to a compositioncomprising graphene, a hydrophobic fluid component at least partiallyimmiscible with an aqueous medium and a graphene dispersing/stabilizingagent at least partially soluble in such a hydrophobic fluid component.Without limitation, such a dispersing agent can comprise an ethylcellulose. In various embodiments, regardless of dispersing agent, sucha hydrophobic fluid component can comprise a component selected fromterpenes, terpene alcohols and combinations thereof. In certain suchembodiments, such a hydrophobic fluid component can comprise aterpineol.

Regardless, a composition of this invention can comprise a grapheneconcentration of greater than about 1 mg/ml. Without limitation as toany particular graphene concentration, such a composition can consist ofunagglomerated graphene flakes, such a morphology as can be evidenced byatomic force microscopy. Regardless, in certain embodiments, such acomposition can be applied to a substrate, such an applied compositioncomprising directionally-aligned graphene flakes.

The present invention can, in part, be directed to a compositecomprising a graphene component coupled to a substrate component, such agraphene component as can comprise nanodimensioned flakes in adisordered network on such a substrate. Alternatively, a graphenecomponent can be considered as comprising an annealation product of anethyl cellulose-stabilized graphene composition. Such an annealationproduct can comprise a transparent conductive graphene thin film, such athin film transparent at visible and/or infrared wavelengths.Regardless, a graphene flake component of such a composite can becharacterized by Raman spectroscopy and a Raman spectrum providing anI(D)/I(G) value less than about 0.75. Without limitation, such asubstrate can comprise a silicon oxide material.

In part, the present invention can also be directed to a method forexfoliating, dispersing or otherwise separating layered, bundled orotherwise agglomerated nanomaterials including, without limitation,carbon nanotubes, such materials as can then be concentrated in a fluidmedium. As but one example, such a method can comprise providing asingle-walled, double-walled or multi-walled carbon nanotube material ina medium comprising an organic solvent at least partially miscible withwater and a carbon nanotube dispersing/stabilizing agent at leastpartially soluble therein; contacting at least a portion of such acarbon nanotube medium with a hydrophobic fluid component; hydratingsuch a medium to concentrate carbon nanotubes within such a hydrophobicfluid component; separating such a nanotube component from such ahydrated medium; and, optionally, iterative contact/separation of such aseparated nanotube-fluid component with additional portions of theaforementioned carbon nanotube medium.

Without limitation, an organic solvent medium can comprise ethanol or asolvent selected from those described herein or as would otherwise beknown to those skilled in the art to be at least partially miscible inan aforementioned hydrophobic fluid component. Likewise, such a carbonnanotube dispersing/stabilizing agent can comprise an ethyl cellulose oran agent as would otherwise be understood by those skilled in the art atleast partially soluble in an aforementioned organic solvent medium.Likewise, a hydrophobic fluid component useful in conjunction with thepresent invention can comprise a component selected from one or moremono-terpene alcohols and various other fluid components describedherein or as would otherwise be understood by those skilled in the artand made aware of this invention. Regardless, a carbon nanotube ink orrelated such fluid component resulting from such iterativeconcentrations can be applied to a suitable substrate, then processed asdescribed herein or as would otherwise be needed for a particularend-use application.

Accordingly, the present invention can also be directed to a compositioncomprising carbon nanotubes (e.g. single-walled carbon nanotubes), ahydrophobic fluid component at least partially immiscible with anaqueous medium and a nanotube dispersing/stabilizing agent at leastpartially soluble in such a hydrophobic fluid component. Withoutlimitation, such a dispersing agent can comprise ethyl cellulose. Invarious embodiments, regardless of dispersing agent, such a hydrophobicfluid component can comprise a component selected from terpenes, terpenealcohols and combinations thereof. In certain such embodiments, such ahydrophobic fluid component can comprise a terpineol. Regardless, acomposition of this invention can comprise a carbon nanotubeconcentration of greater than about 1 mg/ml.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIGS. 1A-B. (A) Digital images of vials of a 1:5 mixture of terpineoland ethyl cellulose stabilized graphene-ethanol solution before andafter water addition. As shown by the colored images, upon the additionof water, the hydrophobic graphene flakes preferentially separate intothe concentrated terpineol fraction, leaving behind an ethanol and watermixture. (B) The concentration factor of graphene (C₀=102.4 μg/mL) isplotted after each solvent exchange concentration and graphene-ethanoladdition step for three iterations.

FIG. 1C. UV-vis absorbance spectra for graphene dispersed in DMF and 1%w/v EC-DMF. Due to the high graphene concentration of the EC-DMFdispersion, the sample was diluted 4× in DMF to obtain a clearabsorbance spectra.

FIGS. 2A-D. (A) Histograms of flake thickness for the initiallyexfoliated and third-iteration concentrated graphene solutions. (B)Digital scanning electron micrograph (SEM) images of a graphene-ethylcellulose nanocomposite fracture surface. (C) Optical transmittanceversus sheet resistance for annealed transparent conductive thin filmsblade coated from the concentrated graphene inks. (D) SEM image of anannealed graphene thin film.

FIGS. 3A-B. (A) Digital AFM image of graphene flakes deposited on SiO₂.(B) Line scan profiles of two deposited graphene flakes, with the largerflake exhibiting edge folding.

FIG. 4. Optical transmittance spectra for the five graphene conductivefilms analyzed.

FIG. 5. Representative Raman spectra of the annealed graphene thin filmand graphene-EC nanocomposite. These spectra were obtained by combiningfive individual spectra from different locations of each film and withthe intensity of the highest peak normalized to unity.

FIG. 6. Digital SEM image of an EC film fracture surface withoutgraphene. The absence of the fracture terraces, in contrast to thoseobserved in FIG. 2B, indicates that the anisotropic fracture behavior ofthe EC-graphene nanocomposite results from aligned graphene flakes.

FIGS. 7A-B. (A) absorbance spectra for dispersions of single-walledcarbon nanotubes, showing enhanced debundling and concentration usingethyl cellulose-ethanol, in accordance with this invention. (Thereference dispersion also illustrates the utility of methylpyrrolidoneas an organic solvent component, in accordance with this invention.) (B)a digital SEM image of an annealed SWCNT thin film.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Without limitation, various embodiments of this invention demonstrate analternative strategy for enhancing graphene exfoliation using apolymer-organic solvent composition. More specifically, as relates tocertain such embodiments, a room-temperature, ultracentrifuge-freeconcentration technique can be used to generate graphene concentrationsin excess of 1 mg/mL in organic solvents that otherwise yield poorgraphene dispersability. The resulting graphene inks are amenable tofurther processing, including casting into aligned graphene-polymernanocomposites and blade coating to form thin films, as a result oftheir low solvent boiling point and non-causticity. Because the presentinvention avoids oxidative conditions, the graphene maintainssuperlative electronic properties, which can be exploited inapplications that require highly conductive, mechanically flexible, andsolution-processable coatings.

Due to the large mismatch between the surface energies of ethanol andgraphite, ethanol is a relatively poor solvent for graphene exfoliation,yielding a post-sedimentation concentration of 1.6 μg/mL. (See,Hernandez, Y.; Lotya, M.; Rickard, D.; Bergin, S. D.; Coleman, J. N.;Langmuir 2010, 26, 3208-3213.) To overcome this limitation, a cellulosicpolymer was used to enhance the ability of ethanol to exfoliate andsuspend graphene flakes. Such polymers include, but are not limited toethyl cellulose, methyl cellulose, hydroxyethyl cellulose, carboxymethylcellulose, and hydroxypropylmethyl cellulose. Using ethyl cellulose(EC), a solution of 50 mg/mL natural graphite flakes in 1% w/vEC-ethanol was sonicated for 3 hr and centrifuged at 7,500 rpm for 4.5hr to remove the fast sedimenting graphite flakes. The resultingsupernatant provides primarily few-layer graphene sheets. Opticalabsorbance was taken to determine the graphene concentration using anabsorption coefficient of 2,460 L/g·m at 660 nm. Without limitation asto any one theory or mode of operation, addition of up to about 1% ormore EC significantly enhanced the graphene exfoliation efficiency byproviding steric stabilization of the exfoliated flakes, yielding apost-sedimentation concentration of 122.2 μg/mL. Despite thisimprovement, still higher concentrations were desired to generategraphene inks that can be easily deposited and patterned.

Towards this end, an iterative solvent exchange was employed as a rapidroom-temperature process to concentrate graphene solutions—without theapplication of centrifugal force. Various hydrophobic fluid solventcomponents at least partially miscible with an organic solvent such anethanol (or e.g., dimethylformamide or methylpyrrolidone), but notmiscible with an aqueous solvent component (e.g., ethanol and water) canbe utilized. In particular, a 1:5 volume ratio solution of terpineol andsedimented graphene solution was prepared and mixed to yield a solutionwith an initial graphene concentration of C₀=102.4 μg/mL. Water, fourtimes the volume of this initial solution, was then added to form ahydrophilic ethanol solution. Again, without limitation to theory ormode of operation, because of the hydrophobicity of the EC-stabilizedflakes, graphene is believed preferentially concentrated into theterpineol band on top of the ethanol-water solution (FIG. 1A). Thisterpineol phase was then harvested and additional sedimented graphenesolution was added for the next concentration iteration. Concentrationfactors, C/Co, were determined after each step through opticalabsorbance for three concentration iterations (FIG. 1B). As expected,the concentration factors correspond roughly to the volumetric reductionof the graphene solution, producing a highly concentrated graphene inkat 1.02 mg/mL after three iterations. Additional iterations of solventexchange yielded diminishing returns as the viscosity of the grapheneink begin to interfere with material separation within the system. Inorder to verify the absence of flake agglomeration during theconcentration process, atomic force microscopy was performed on over 140flakes deposited from the sedimented graphene solution and the thirditeration graphene ink. Both media exhibited similar flake thicknesshistograms peaked at approximately 1.6-1.8 nm (FIG. 2A), suggestingminimal graphene agglomeration during the concentration process.

Graphene-polymer nanocomposites were solution cast from these grapheneinks. The height reduction associated with anisotropic volumecontraction during solvent evaporation resulted in the directionalalignment of the graphene flakes within the nanocomposite. In FIG. 2B,this alignment is evident on the fracture surface in the form of shearedterraces orthogonal to the direction of the volumetric contraction. Thelack of protruding graphene flakes on the fracture surface is not onlyindicative of flake alignment but also suggests strong interactionsbetween the polymer and graphene.

The electrical properties of thin films derived from the concentratedgraphene ink were assessed via transparent conductor measurements. Dueto their enhanced rheology, film forming capability, and dispersionstability, EC-stabilized graphene inks are amenable to blade coatingonto a broad range of substrates. For example, graphene inks were bladecoated onto glass slides at varying thicknesses, annealed at 400° C. for30 min in air, and rinsed with acetone to produce transparent conductivethin films. Four point probe measurements of the film sheet resistanceindicate that their electrical performance compare favorably to filmsdeposited by vacuum filtration from sedimented surfactant graphenesolutions (FIG. 2C). Electron microscopy performed on these conductivegraphene thin films (FIG. 2D) reveals a disordered network of grapheneflakes with lateral dimensions ranging from approximately 50-400 nm.Raman spectra provide further evidence that these graphene thin filmspossess low defect densities and negligible oxidation.

As demonstrated, efficient graphene exfoliation can be achieved inethanol through polymeric stabilization using ethyl cellulose. Theresulting graphene solutions can be concentrated via rapid,room-temperature, ultracentrifugation-free iterative solvent exchange,ultimately yielding stable graphene inks at mg/mL levels. Theoutstanding processability and electrical properties of the resultinginks enable the straightforward production of functional graphene-basedmaterials including highly anisotropic polymer nanocomposites andtransparent conductive thin films. Such results can promote ongoingefforts to understand and exploit solution-processable pristine graphenefor fundamental studies and device applications.

Examples of the Invention.

The following non-limiting examples and data illustrate various aspectsand features relating to the methods and/or compositions of the presentinvention, including the preparation and use of concentrated graphenesolutions, compositions and related composites, as are described herein.In comparison with the prior art, the present methods provide resultsand data which are surprising, unexpected and contrary thereto. Whilethe utility of this invention is illustrated through the use of severalgraphene dispersion agents and hydrophilic organic solvents, togetherwith several hydrophobic fluid components which can be used therewith,it will be understood by those skilled in the art that comparableresults are obtainable with various other dispersion agents andhydrophilic or hydrophobic solvents, as are commensurate with the scopeof this invention.

Example 1a

Exfoliation and Sedimentation Processing of Graphene.

2.5 g of natural graphite flake (3061 grade, Asbury Graphite Mills) wasadded to 50 mL of 1% w/v ethyl cellulose (EC) (Aldrich) ethanol (EtOH)solution inside a plastic 50 mL centrifuge tube (note that Aldrich doesnot explicitly provide the molecular weight of its EC; rather, theviscosity is specified (e.g., 4 cP) when the EC is loaded at 5% w/v in80:20 toluene:ethanol). Two tubes containing this mixture weresimultaneously sonicated in a Bransonic 3510 tabletop ultrasonic cleanerfor 3 hr at 40 kHz and 100 W. In order to efficiently sediment out thegraphite flakes, the centrifugation was performed in a two-step process.First, the sonicated graphene dispersions were centrifuged in a largevolume centrifuge (Beckman Coulter Avanti J-26 XP Centrifuge) for 10 minat 7,500 rpm to remove the fast sedimenting graphite flakes. Thesupernatant was then decanted from each 50 mL centrifuge tube andcombined. A second sedimentation step was then performed on thiscombined solution in two 250 mL tubes for 4.5 hr at 7,500 rpm or anaverage relative centrifugal force (RCF) of 6,804 g.

Example 1b

Thermal Stability of Polymer Enhanced Graphene Dispersions.

Experiments were undertaken to highlight the thermal stability ofEC-based graphene dispersions, of the sort discussed above, especiallyin comparison to traditional surfactant-based dispersions. Here,graphene dispersions in 1% w/v EC-EtOH and 1% w/v sodium cholate-water(SC—H₂O, prior art) were produced using the sonication andcentrifugation procedures detailed above. Both dispersions were thenconcentrated to ˜1 mg/mL via thermal evaporation.

At elevated temperatures, graphene flakes in the SC-based dispersionagglomerate rapidly to form precipitates, while the EC-based dispersionremains well dispersed. To quantify their thermal stabilities, bothconcentrated dispersions were diluted to 0.1 mg/mL and centrifuged at15,000 rpm for 1 min. The UV-vis absorbance spectra for theirsupernatants were then obtained. Using the same absorbance coefficientdiscussed above (2460 L/g·m at 660 nm), it was determined that 97.7% ofthe graphene remained suspended in the EC-EtOH medium, while only 18.1%remained suspended in the SC—H₂O solution. The stability of thesepolymer-based graphene dispersions can be exploited in subsequentpost-synthetic processing.

Example 1c

Enhanced Graphene Production Efficiency in DMF.

Improvement in graphene production is also demonstrated by adding EC todimethylformamide (DMF), which has moderate intrinsic graphenesolubility. In this case, natural graphite was bath sonicated for 3 h at50 mg/mL in both DMF and 1% w/v EC-DMF. After centrifugation at 7500 rpmfor 4.5 h to remove the thick graphite flakes, UV-vis absorbance spectrawere taken to assess their graphene concentrations (FIG. 1C).

Using an absorbance coefficient of 2460 L/g·m at 660 nm, the grapheneconcentration for the DMF and EC-DMF dispersions were determined to be14.1 and 82.8 μg/mL, respectively. (See, Hernandez, Y.; Nicolosi, V.;Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.;Byrne, M.; Gun′Ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.;Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A.C.; Coleman, J. N., Nat. Nanotechnol. 2008, 3, 563-568). It followsthat, the addition of 1% w/v EC to DMF yielded a 5.9-fold improvement inthe graphene exfoliation/production efficiency. Overall, improving thegraphene exfoliation efficiency in organic solvents with moderate tohigh intrinsic graphene solubilities can both reduce material waste andbenefit printed electronic and related applications where highergraphene-to-dispersant ratios are required.

In accordance with this invention, without limitation, various otherC₂-C₅ alcohols, esters, ethers, ketones and amides can be used, inconjunction with a cellulosic polymer, to suspend and exfoliategraphene.

Example 2

Graphene Concentration via Iterative Solvent Exchange.

To ensure proper hydrophobic phase separation, water, in excess of fourtimes the volume of the starting graphene solution, is added. A briefsonication step, of approximately 1 min, is also performed after eachgraphene concentration and graphene addition step to facilitate phaseseparation and solution mixing.

Example 3

SiO₂ Graphene Deposition.

Graphene flakes from both the sedimented graphene solution andthird-iteration concentrated graphene solution were deposited onto 100nm thick oxide silicon wafers for imaging. The wafers were firstsubmerged in 2.5 mM 3-aminopropyl triethoxysilane aqueous solution tofunctionalize the surface with a hydrophobic self-assembling monolayerfor 30 min. The substrates were then rinsed with water and dried under astream of N₂. Both graphene solutions were then diluted to approximately0.02 mg/mL in ethanol after which a drop of each was placed onto thefunctionalized wafers for 10 min. The drops were then blown off under astream of N₂, and the wafer was rinsed with water. To remove theresidual EC, the wafers were annealed for 20 min at 400° C. in air.

Example 4

Atomic Force Microscopy Thickness Measurements.

All atomic force microscopy (AFM) images were obtained using a ThermoMicroscopes Autoprobe CP-Research AFM in tapping mode using cantilever Bon MikroMasch NSC NSC36/Cr-AuBS probes. 2 μm×2 μm images were collectedusing identical scanning parameters. Flake thicknesses were determinedusing line-scan thickness profiles across flakes larger than 5,000 nm²while avoiding regions where EC residues were present. (FIGS. 3A-B) 146flakes were analyzed on the wafer deposited with the sedimented graphenesolution, and 156 flakes were analyzed for the wafer deposited with thethird-iteration concentrated graphene solution.

Example 5

Thin Film Deposition.

Graphene-EC and graphene thin films were blade coated from concentratedgraphene inks onto glass slides using either 1 or 2 layers of 3M ScotchMagic Tape (about 30-about 40 μm per layer) as masks. In order tooptimize ink rheology for uniform film deposition, 10% w/v EC (Aldrich,22 cP, 5% in toluene:ethanol 80:20) in ethanol was added to the grapheneink. The modified graphene ink was deposited into 2 cm×2 cm squares on2.54 cm×2.54 cm silica glass slides. To obtain films with differentoptical densities, select films were also spun at 10,000 rpm for 3 min.These films were then allowed to dry overnight, and the mask was removedto obtain a transparent graphene-polymer film (not shown). Graphene thinfilms require an additional annealing step, performed for 30 min at 400°C. in air, to remove the EC and enhance flake-to-flake contact. Afterannealing, these graphene thin films were rinsed in acetone beforeoptical transmittance and four point probe measurements.

Example 6

Optical Absorbance and Transmittance Measurement.

Optical absorbance measurements to determine graphene solutionconcentrations and transmittance measurements for transparent conductivegraphene thin films were performed using a Varian Cary 5000spectrophotometer. Background from the optical cuvette, EC-ethanolsolution, and glass slide were subtracted from the spectra of thegraphene dispersions and films. Due to their high absorbance,concentrated graphene solutions were diluted either 4× or 10× to ensurethat the optical absorbance was within the detector limits. As expected,the graphene thin films of the preceding example provide featurelessoptical absorbance spectra with high transparency at visible andinfrared wavelengths (FIG. 4).

Example 7

Raman Spectroscopy of the Graphene Films. Raman spectroscopy wasobtained using a Renishaw inVia Raman microscope with an excitationwavelength of 514 nm. Five spectra were obtained on different areas ofthe annealed graphene film and the graphene-EC nanocomposite using abeam size of 1-2 μm, allowing multiple flakes to be probed in eachmeasurement. These spectra showed minimal variation across differentlocations and were combined to form a representative Raman spectrum forthe entire film (FIG. 5). Typical Raman spectra for the annealedgraphene film exhibit four primary peaks: the G band at ˜1,590 cm⁻¹, 2Dband at ˜2,700 cm⁻¹, and the disorder-associated D and D′ bands at˜1,350 cm⁻¹ and ˜1,620 cm⁻¹ respectively. The intensity ratio of the Dand G bands, I(D)/I(G), is a measure of the level of defects that areintroduced during the sonication and annealing processes. The I(D)/I(G)value for the annealed graphene film was ˜0.38, significantly less thanreported values for surfactant exfoliated graphene solutions with asimilar size distribution (˜0.93) (see, Green, A. A.; Hersam, M. C.;Nano Lett. 2009, 9, 4031-4036) and heavily reduced graphene oxide(˜0.82) (Gao, W.; Alemany, L. B.; Ci, L.; Ajayan, P. M. Nature Chem.2009, 1 (5), 403-408) but higher than that for larger-sized solventexfoliated graphene flakes. (See, Hernandez, Y.; Nicolosi, V.; Lotya,M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne,M.; Gun′Ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.;Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A.C.; Coleman, J. N. Nat. Nanotechnol. 2008, 3, 563-568.) The measuredvalue of ˜0.38 indicates that large quantities of defects or oxidationwere not introduced during exfoliation and annealing.

Example 8

Nanocomposite Fracture Surface.

The graphene-EC and graphene-free EC films were fractured using shearingforces applied orthogonally to the planes of the films. The fracturedsurfaces were then analyzed using SEM to gauge the adhesion strength ofgraphene to EC and orientation of graphene flakes. (See, FIG. 6.)

Example 9

Scanning Electron Microscopy. Scanning electron microscopy of thetransparent conductive graphene thin films and fracture surfaces ofgraphene-EC nanocomposites was performed on a Hitachi 4800 scanningelectron microscope using a 1 kV accelerating voltage.

Example 10

Dispersion and iterative solvent exchange can be used concentrate fluidmedia comprising other nanodimensioned materials, such as single-walledcarbon nanotubes, using procedures analogous to those described inexamples 1-2. For instance, single walled carbon nanotubes (SWCNTs) weredispersed in 1% EC-EtOH via 1 h horn sonication and 4.5 h centrifugationat 7500 rpm. Compared to a reference 0.04 mg/mLSWCNT/N-methylpyrrolidone (NMP) dispersion, without EC, theconcentration of the 1% EC-EtOH dispersion was determined to be around0.75 mg/mL (see, FIG. 7A). Solvent exchange with terpineol provided aconcentrated SWCNT-EC ink. Likewise, substrate deposition and materialcharacterization can be accomplished, using techniques of sort describedin examples 3-9. A transparent SWCNT thin film was prepared by bladecoating and annealing the aforementioned ink at 400° C. in air for 30minutes. An SEM image of the annealed SWCNT thin film is shown in FIG.7B.

As demonstrated, above, the present invention provides a method forenhanced concentration of graphene, carbon nanotubes, and relatednanomaterials to provide, in particular, graphene concentrationsheretofore unrealized in the art. Such techniques are rapid andscalable, making more readily available the various mechanical, chemicaland electronic attributes of such materials over a wide range of end-useapplications.

We claim:
 1. A method of using a cellulosic polymer to provideconcentrated graphene in preparation of a graphene ink composition, saidmethod comprising: exfoliating a graphene source material with a mediumcomprising an organic solvent at least partially miscible with water anda dispersing agent comprising a cellulosic polymer at least partiallysoluble in said organic solvent, said graphene source material notoxidized; contacting at least a portion of said exfoliated graphenemedium with a hydrophobic fluid component; and hydrating said exfoliatedgraphene medium to concentrate said exfoliated graphene in saidhydrophobic fluid component, said concentrated graphene medium isachieved without application of centrifugal force.
 2. The method ofclaim 1 wherein said dispersing agent comprises an ethyl cellulose. 3.The method of claim 1 wherein said organic solvent is selected fromC₂-C₅ alcohols, esters, amides, ethers and ketones and combinationsthereof.
 4. The method of claim 3 wherein said organic solvent isselected from ethanol, methylpyrrolidone and dimethylformamide.
 5. Themethod of claim 1 wherein said hydrophobic fluid component is selectedfrom terpenes, terpene alcohols and combinations thereof.
 6. The methodof claim 5 wherein said hydrophobic fluid component comprises aterpineol.
 7. The method of claim 1 wherein said graphene sourcematerial is a graphite.
 8. The method of claim 1 further comprisingiterative separation of said graphene-hydrophobic fluid component fromsaid organic solvent and contact with another portion of said exfoliatedgraphene medium, to provide a graphene ink composition.
 9. The method ofclaim 8 comprising application of said graphene ink medium on asubstrate.
 10. The method of claim 9 wherein said graphene ink medium isannealed to remove said dispersing agent.
 11. A method of using acellulosic polymer to prepare a graphene ink composition, said methodcomprising: exfoliating a graphene source material with a mediumcomprising an organic solvent at least partially miscible with water anda dispersing agent comprising a cellulosic polymer at least partiallysoluble in said organic solvent, said graphene source material notoxidized; contacting at least a portion of said exfoliated graphenemedium with a hydrophobic fluid component; hydrating said exfoliatedgraphene medium to concentrate said exfoliated graphene in saidhydrophobic fluid component, said concentrated graphene medium isachieved without application of centrifugal force; and iterativelyseparating said graphene-hydrophobic fluid component from said organicsolvent and contact with another portion of said exfoliated graphenemedium, to provide a graphene ink composition.
 12. The method of claim11 wherein said dispersing agent comprises an ethyl cellulose.
 13. Themethod of claim 11 wherein said organic solvent is selected from C₂-C₅alcohols, esters, amides, ethers and ketones and combinations thereof.14. The method of claim 13 wherein said organic solvent is selected fromethanol, methylpyrrolidone and dimethylformamide.
 15. The method ofclaim 11 wherein said hydrophobic fluid component is selected fromterpenes, terpene alcohols and combinations thereof.
 16. The method ofclaim 15 wherein said hydrophobic fluid component comprises a terpineol.17. The method of claim 11 wherein said graphene source material is agraphite.
 18. The method of claim 11 comprising application of saidgraphene ink medium on a substrate.
 19. The method of claim 18 whereinsaid graphene ink medium is annealed to remove said dispersing agent.